The growth in personal computing coupled with the availability of low-cost digital network services has led to significant increases in user demand for high-speed digital connectivity. Of particular interest to personal computer users is connectivity at one of the newest available data rates of 19.2 kbps. Almost all existing personal computers can readily utilize digital 19.2 kbps service with little or no additional software or hardware investment.
From a service provider's perspective, the growth in use of 19.2 kbps connectivity requires a thorough support plan that includes testing and maintenance of local loops. For a typical four-wire circuit, testing of local loops is performed by so-called loopbacks, in which test signals from a common network test center are transmitted as data bytes to an office channel unit (OCU) and from there, over the local 4-wire loop to a customer channel unit (CSU) at the customer premises equipment (CPE). For testing purposes, the signal received at the CPE is immediately returned back to the test equipment by the CSU; hence, the term "loopback." Two different types of loopback tests are performed, "non-latching" and "latching" loopbacks. Non-latching loopbacks are so labelled because the loopback is sustained by the test system transmitting a loopback code at least every other byte interval. Typically, testing is performed by interleaving random test data and loopback codes on an alternating basis and monitoring the results.
Latching loopback testing is implemented in a different fashion. A latching loopback, once activated, using unique sequence of codes, remains in place until a latching loopback release sequence is transmitted by the test system. The use of latching loopback then will allow the test system to send a continuous random sequence without the need to interleave a loopback code to sustain the loopback. Latching loopbacks are often used to perform long term stress tests. Several different stress patterns are currently defined and used as part of current digital testing. These include pseudo-random patterns of 2047 and 511 and DDS stress patterns 1-4.
Typically, all network services, including 19.2 kbps, are tested from a common test center using standard test equipment. Using test equipment that has been upgraded to include test procedures specific to 19.2 kbps will provide more than adequate test support for both latching and non-latching loopbacks. However, the test systems of all network service providers have not been upgraded to include 19.2 kbps capability. Under these circumstances, a need exists for an alternative test procedure and apparatus to provide the necessary test and maintenance support for a 19.2 kbps service operable with test equipment which has not been upgraded.